The formation of the Yichun Ta-Nb deposit, South China, through fractional crystallization of magma indicated by fluid and silicate melt inclusions

Abstract

The Yichun Ta-Nb deposit, which is located in Jiangxi Province, South China, can be divided into four lithological zones (from bottom upward): two-mica granite, muscovite granite, albite granite, and lepidolite-albite granite zones. It remains controversial whether these distinct vertical zones were formed through late magmatic-hydrothermal metasomatic alteration or fractional crystallization of magma. To investigate the evolution mechanism of rock- and ore-forming fluid in this deposit, we studied fluid and melt inclusions in quartz and lepidolite in these four granite zones. These fluid inclusions are mainly composed of H2O-NaCl, and have homogenization temperatures ranging from 160°C to 240°C, with densities between 0.86 and 0.94g/cm3 and salinities between 0.5 and 6.5wt% NaCl equivalent. Raman spectroscopic analyses showed that the daughter minerals contained in silicate melt inclusions are mainly quartz, lepidolite, albite, muscovite, microcline, topaz, and sassolite. From the lower to upper granite zones, the albite contents in silicate melt inclusions increase, while the muscovite contents decrease gradually until muscovite is substituted by lepidolite in the lepidolite-albite granite zone. Additionally, the calculated densities of the silicate melt inclusions exhibit decreasing trends from bottom upward. The total homogenization temperatures of silicate melt inclusions, which were observed under external pressures created in the sample chamber of a hydrothermal diamond-anvil cell, decreased from 860°C in the lower lithological zone to 776°C in the upper lithological zone, and the initial melting temperatures of solid phases were 570–710°C. The calculated initial H2O contents of granitic magma showed an increasing trend from the lower (∼2wt% in the two-mica granite zone) to the upper granitic zones (∼3wt% in the albite granite zone). All of these features illustrate that the vertical granite zones in the Yichun Ta-Nb deposit formed through the continuous fractional crystallization of the granitic magma. Additionally, the low H2O contents and gradual enrichment of incompatible elements (F, B, Li, Ta, Nb, etc.) in the residual granitic magma favored the formation of a granite-type Ta-Nb deposit.